1. Field of the Invention
The present invention relates to a droplet ejection head and an image recording apparatus, and more specifically to a droplet ejection head and an image recording apparatus having an improved configuration of an electromechanical transducer and a mechanoelectrical transducer for determining oscillation in a pressure chamber of the droplet ejection head using piezoelectric elements.
2. Description of the Related Art
One known example of an image recording apparatus is an inkjet recording apparatus having an inkjet head (print head) with an array of multiple nozzles (image recording elements), in which an image is formed on a recording medium by ejecting ink from the nozzles while moving the inkjet head and the recording medium relative to each other.
There are various ink ejection systems for the inkjet head of such an inkjet recording apparatus. Known examples include a piezoelectric system in which a diaphragm defining a pressure chamber is deformed by the deformation of a piezoelectric element (piezoelectric ceramics) to vary the capacity of the pressure chamber, ink is introduced into the pressure chamber through an ink supply channel during the capacity increase of the pressure chamber, and the ink in the pressure chamber is ejected as droplets from a nozzle when the capacity of the pressure chamber decreases; and a thermal inkjet system in which the ink is heated to create air bubbles and is ejected by the energy of expansion when the air bubbles increase in size.
For example, an inkjet head that uses a piezoelectric element has a stacked structure formed by stacking on a substrate a piezoelectric element, a diaphragm, a flow path plate with an ink supply channel and a pressure chamber formed therein, and a nozzle plate (orifice plate) with an ink ejection hole formed therein.
In such an inkjet head that uses a piezoelectric element, when air sometimes gets mixed in the ink held in the head, the variation of the pressure chamber capacity is absorbed by the air bubbles formed by this air, so that sufficient pressure cannot be applied to the ink any longer, and ink ejection thus becomes incomplete. Moreover, ink droplets cannot be ejected when the nozzle is clogged with dirt or the like, or when the ink does not fill the pressure chamber. In such a case, the state of oscillation in the pressure chamber differs from that during normal ink droplet ejection. Various methods have been proposed for determining the state of oscillation in the pressure chamber in order to detect ink ejection failure or incomplete ejection.
Japanese Patent Application Publication No. 55-118878 discloses an ink injector including an ink chamber (pressure chamber), which has an ink supply port through which ink is supplied from an ink tank and an ink droplet ejection port through which the ink is atomized and ejected. Part of a wall surface defining the ink chamber is provided with a vibrator that is displaced and varies the capacity in the ink chamber in response to an electrical signal, and another part of the wall surface is provided with a detector for determining the displacement. When an abnormal displacement is detected by the detector, either the operator is informed so as to perform an air discharge operation, or an automatic air discharge operation for forcing a predetermined amount of ink out of the ink chamber and simultaneously discharge of the admixed air is performed, whereby the problem of ink ejection failure is resolved. However, this composition has problems in that a sufficient surface area is required for the mechanoelectrical transducer used for determination to be bonded onto the diaphragm, so that the surface area occupied by the electromechanical transducer used for drive must be decreased in proportion to the space to which the mechanoelectrical transducer used for determination is to be attached, which is inconvenient in terms of driving efficiency.
Japanese Patent Application Publication No. 63-122549 discloses that a drive pulse voltage is sent to all of a plurality of electromechanical transducers provided to the side wall of the ink chamber to eject ink from the ink chamber during the recording operation, and that during an abnormality detecting process, a determination pulse voltage not causing ink ejection is sent only to some of the plurality of electromechanical transducers, and the rest of the electromechanical transducers are switched so as to be connected to an abnormality determination circuit, whereby abnormalities such as the presence of air bubbles are detected by determining the oscillation state in the pressure chamber. However, this composition has problems in that since some of the plurality of electromechanical transducers are used as mechanoelectrical transducers for determination by the switching circuit, the determination sensitivity is insufficient because the oscillation generated by some of the electromechanical transducers is determined on the adjacent diaphragm, and furthermore, the electromechanical transducer cannot be used for drive and ink cannot be ejected during the abnormality detecting process.